This invention relates to a digital signal processing circuit used in voice synthesis, musical synthesis, and the like, and more particularly, to a digital processing circuit with a rounding function for the results of a multiplication function.
The omission of several lower bits has been employed for rounding the results of a multiplication function. The rounding attenuates the true value. Because of this fact, when one attempts to generate an attenuated sine-wave of a desired frequency utilizing the impulse response characteristic of a lattice filter, it is impractical to use the sine-wave as the scale signal in a musical tone because the attenuation time constant of the sine-wave is too great. Recently, in musical tone synthesis using the lattice filter, the raising of several lower bits to a unit has been proposed for rounding the results of multiplication. The technology concerning raising lower bits to a unit is described in a paper by Hibino et al, entitled "Generation of Musical Tones in PARCOR Speech Synthesis LSI" in Transactions of the Committee on Speech Research, The Acoustical Society of Japan, S82-04 (Apr. 26, 1982), pp 25-32. It is noted, however, that the raising to a unit described in the above article involves the following problems. To discuss the problems, it is assumed that the result of multiplication of M-bit data is Z, and the value of the M-bit data after the N lower bits of the M-bit data have been raised to a unit is Z'. It is further assumed that both Z and Z' are expressed in 2's complement format. The article describes that if Z is positive or zero, all of the N lower bits are made binary "1"s, and if Z is negative, all of these bits are made binary "0"s. If .vertline.Z.vertline..ltoreq.1, a relationship between Z and Z' can be plotted, as shown in FIG. 1, in which the X-axis represents Z and the Y-axis represents Z'. In FIG. 1, a broken line depicts the Z--Z' relationship when the raising to a unit is not performed. A solid line indicates the relationship when the raising to a unit is performed. As seen in FIG. 1 in the latter case, Z' changes stepwise with respect to Z.
Let us consider the mean value, i.e., the DC component of the signal, after having been raised to a unit, when a signal with no DC component is input to the circuit with the input-output characteristic of FIG. 1. Table 1 shows the relationship between levels of the signal after having been raised to a unit and probabilities of these level occurrences. In Table 1, .DELTA.=2.sup.-(M-N-1) and Q=2.sup.-N. The DC component of the signal after having been raised to a unit, which is the sum of the products of the respective signal levels and the corresponding probabilities in Table 1, is expressed by -.DELTA./2.sup.N+1. This indicates that the signal after having been raised to a unit includes the negative DC component.
TABLE 1 ______________________________________ Signal level after raising Occurrence to a unit probability ______________________________________ (2.sup.M-N-1 -Q) .multidot. .DELTA. 2.sup.N-M (2-Q) .multidot. .DELTA. 2.sup.N-M (1-Q) .multidot. .DELTA. 2.sup.N-M 0 0 -.DELTA. 2.sup.N-M -2 .multidot. .DELTA. 2.sup.N-M -2.sup.M-N-1 .multidot. .DELTA. 2.sup.N-M ______________________________________ DC component: -.DELTA./2.sup.N+1
Therefore, when the rounding process in which the multiplication result is raised to a unit is applied to voice synthesis, musical tone synthesis, etc., in which digital filters, lattice filters 2'having many multiplying circuits are used, a considerable amount of DC component will occur. This degrades the S/N performance of the device involved.